1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to packet transmission scheduling of the High Speed Downlink Packet Access (HSDPA) system operated in a UMTS terrestrial radio access network (UTRAN).
2. Background of the Related Art
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from a Global System for Mobile Communications (GSM) and a European style mobile communication standard. It is intended to provide improved mobile communication services based on a GSM core network (CN) and Wideband Code Division Multiple Access (WCDMA) access technology.
For the purpose of making a standard for third generation mobile communication systems (IMT-2000 systems) based on GSM core network and WCDMA radio access technology, a group of standard developing organizations, including ETSI of Europe, ARIB/TTC of Japan, T1 of U.S., and TTA of Korea, established the Third Generation Partnership Project (3GPP).
For the purpose of efficient management and technological development, five Technical Specification Groups (TSGs) were organized under the 3GPP in consideration of network construction factors and their operations.
Each TSG is responsible for approving, developing, and managing specifications related to a pertinent area. Among them, the Radio Access Network (RAN) group has developed functions, requirements, and interface specifications related to UE and UMTS terrestrial radio access network (UTRAN) in order to establish a new radio access network specification to the third generation mobile communication system.
The TSG-RAN group consists of one plenary group and four working groups. Working Group 1 (WG1) has been developing specifications for a physical layer (Layer 1) and WG2 has been specifying functions of a data link layer (Layer 2) between UE and UTRAN. In addition, WG3 has been developing specifications for interfaces among Node Bs (the Node B is a kind of base station in the wireless communications), Radio Network Controllers (RNCs), and the core network. Lastly, WG4 has been discussing requirements for radio link performance and radio resource management.
FIG. 1 illustrates a structure of the UTRAN defined in 3GPP. As shown in FIG. 1, the UTRAN 110 includes at least one radio network sub-systems (RNSs) 120 and 130. Each RNS 120, 130 includes an RNC 121, 131 and at least one or more Node Bs 122, 123, 132, 133. For example, Node B 122 is managed by RNC 121, and receives information transmitted from the physical layer of the UE 150 through an uplink channel and transmits a data to the UE 150 through a downlink channel.
Accordingly, the Node B acts as an access point of the UTRAN from the UE point of view.
The RNCs 121 and 131 allocate and manage radio resources of the UMTS and are connected to a suitable element of the core network 140 depending on types of services provided to users.
For example, the RNCs 121 and 131 are connected to a mobile switching center (MSC) 141 for a circuit-switched communication such as a voice call service, and are connected to a Serving GPRS Support Node (SGSN) 142 for packet switched communication such as a wireless Internet service.
The RNC in charge of a direct management of the Node B is called a Control RNC (CRNC). The CRNC manages common radio resources.
On the other hand, the RNC that manages dedicated radio resources for a specific UE is called a Serving RNC (SRNC). The CRNC and the SRNC can be co-located in the same physical node. However, if the UE has been moved to an area of a new RNC that is different from SRNC, the CRNC and the SRNC may be located at physically different places.
There is an interface that can operate as a communication path between various network elements. The interface between a Node B and a RNC is called a lub interface, and an interface between RNCs is called a lur interface. And an interface between the RNC and the core network is called a lu interface.
FIG. 2 illustrates a protocol structure of a radio interface protocol defined in the 3GPP. As shown in FIG. 2, the radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer, and is vertically divided into a control plane for transmission of control information (signaling) and a user plane for transmission of data information.
The user plane is a region through which user traffic such as voice information or IP (Internet Protocol) packets is transmitted, and the control plane is a region through which control information required for network maintenance and management is transmitted.
Of the layers, the physical layer (PHY) handles transmission of data using a wireless physical channel between the UE and the UTRAN. The typical functions of the physical layer include data multiplexing, channel coding, spreading, and modulation.
The physical layer exchanges information with a Medium Access Control (MAC) layer through a transport channel. The transport channel is classified into a dedicated transport channel and a common transport channel depending on whether its use is dedicated to one UE or whether it is shared among several UEs.
The MAC layer transmits data by using a suitable mapping between logical channels and transport channels. The MAC layer is internally divided into two sub-layers, to with a MAC-d sub-layer which manages the dedicated transport channel, and a MAC-c/sh sub-layer which manages the common transport channel. The MAC-d sub-layer is located in the SRNC and the MAC-c/sh sub-layer is located in the CRNC.
There are various kinds of logical channels according to what kind of information the channel carries. The logical channel can be divided into two channels. One logical channel is a control channel for transmission of the control plane information and the other is a traffic channel for transmission of the user plane information.
The radio link control (RLC) layer is responsible for reliable transmission of RLC protocol data units (PDUs). The RLC may segment or concatenate RLC service data units (SDUs) delivered from the higher layer. If the RLC PDUs are ready, they are delivered to the MAC layer and transmitted sequentially to the other node (UE or UTRAN). Sometimes, the RLC PDU can be lost during the transmission. In this case, the lost PDU can be retransmitted. The retransmission function of the RLC layer is called an Automatic Repeat reQuest (ARQ).
The RLC layer may include several RLC entities. Each of them performs an independent radio link control function. The operation mode of each RLC entity is one of transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM) depending on the adopted functions.
The Packet Data Convergence Protocol (PDCP) layer is positioned over the RLC layer and efficiently transmits data of network protocols such as IPv4 or IPv6. For example, a header compression method in which header information of a packet is reduced can be used. The PDCP layer may include several independent PDCP entities like the RLC layer. The Broadcast/Multicast Control (BMC) layer is responsible for transmitting broadcast messages from a Cell Broadcast Center (CBS) positioned at a core network. The primary function of BMC is to schedule and transmit cell broadcast messages destined for a UE. The BMC layer, in general, uses an RLC entity operated in the unacknowledged mode in order to transmit broadcast messages.
Finally, the Radio Resource Control (RRC) layer is a layer defined in the control plane. The RRC performs functions of establishment, reestablishment, and release of radio resources. In addition, the RRC layer can exchange control information between UE and UTRAN using RRC messages.
The maximum transmission rate of UMTS is 2 Mbps in the indoor and pico-cell environment, and 384 Kbps in the outdoor environment. However, as wireless Internet services have become popular, various services require higher data rates and higher capacity. Although UMTS has been designed to support multimedia wireless services, the maximum data rate of 2 Mbps is not enough to satisfy the required quality of services. Therefore, the 3GPP is conducting research directed to providing an enhanced data rate and radio capacity. One result of the research is the High Speed Downlink Packet Access (HSDPA). The purpose of the HSDPA system is to provide a maximum data rate of 10 Mbps and to improve the radio capacity in the downlink.
Various techniques in the HSDPA system include Link Adaptation (LA) and Hybrid Automatic Repeat reQuest (HARQ).
In the LA method, the UTRAN can choose the appropriate modulation and coding scheme (MCS) according to the channel condition. For example, if the channel condition is good, LA uses 16 Quadrature Amplitude Modulation (QAM) to increase the throughput. If a channel condition is not as good, however, LA uses Quadrature Phase Shift Keying (QPSK) to increase the probability of success.
The HARQ method retransmits lost packets, but the exact operation is different than the retransmission method in the RLC layer. If one packet is corrupted during transmission, HARQ transmits another packet that contains the additional information for recovery. The retransmitted packet and the original packet are combined in the receiver. The retransmitted packet may contain the same information as that of the previously transmitted data, or may contain any additional supplementary information for data recovery.
Since the HSDPA system is an evolutional form of the UMTS system, the conventional UMTS network needs to be maintained as much as possible to support backward compatibility and to reduce the cost of network deployment. However, some minor changes are inevitable.
To reduce the impact of the changes, most of the features are supported in Node B. This means that other parts of the UMTS network will not be affected. Accordingly, some functions in Node B need to be changed and some MAC functions are transferred from RNC. The MAC functionalities constitute a new MAC sublayer in Node B and it is called “MAC-hs” sublayer.
The MAC-hs sublayer is placed above the physical layer, and performs packet scheduling and various other functions (including HARQ and LA). In addition, the MAC-hs sublayer manages a transport channel called an HSDPA—Downlink Shared Channel (HS-DSCH), which is used to deliver data from the MAC-hs sublayer to the physical layer.
FIG. 3 shows a structure of a radio interface protocol for the HSDPA system. As shown in FIG. 3, the MAC-hs sublayer is placed over the physical layer (PHY) in the Node B. In both the UE and UTRAN, the MAC-hs sublayer transfers data to the upper layer through MAC-c/sh and MAC-d sublayers. The MAC-c/sh and the MAC-d sublayers are located in the CRNC and the SRNC, respectively, as in the related art system. In FIG. 3, an HS-DSCH Frame Protocol (FP) delivers the HSDPA data on the lub or the lur interface.
FIG. 4 illustrates the structure of the MAC layer in the HSDPA system. As shown in FIG. 4, the MAC layer is divided into a MAC-d sublayer 161, a MAC-c/sh sublayer 162, and a MAC-hs sublayer 163. The MAC-d sublayer 161 is in the SRNC and manages dedicated logical channels. The MAC-c/sh sublayer 162 is located in the CRNC and manages a common transport channel. The MAC-hs sublayer 163 is located in the Node B and manages the HS-DSCH.
In the HSDPA system, the MAC-c/sh sublayer 162 controls the data flow between the MAC-d sublayer 161 and the MAC-hs sublayer 163. The flow control function is used to prevent data from being discarded during network congestion, and to reduce the time delay of signaling signals. The flow control can be performed independently according to a priority of data transmitted through each HS-DSCH.
The HARQ function, as stated previously, improves the efficiency of data transmission. In the Node B, the MAC-hs sublayer 163 contains one HARQ block that supports the HARQ function. The HARQ block includes several HARQ entities for controlling a HARQ operation of each UE. There is one HARQ entity for each UE in the HARQ block.
Moreover, there are several HARQ processes inside each HARQ entity. Each HARQ process is used for transmission of the “data block,” which is composed of one or more MAC MAC-hs SDUs. The data block is processed one by one in the HARQ process.
If the specific data block is successfully transmitted, the HARQ process can treat another data block. If the transmission fails, the HARQ process retransmits the data block until the data block is either successfully transmitted or discarded. The number of MAC-hs SDUs constituting the data block differs depending on the status of the radio channel. If the channel is in a good condition, it can transmit more MAC-hs SDUs. Conversely, if the channel is in a bad condition, it can transmit fewer MAC-hs SDUs, and therefore a relatively small number of MAC-hs SDUs comprises a data block.
The scheduling block determines the size of the data block based on the information (channel condition) from the physical layer. Each data block can be transmitted in the unit of Transmission Time Interval (TTI) which is 2 ms in the HSDPA system. In addition, the scheduling function of the Node B determines the order of the data transmission according to the priorities of data. The scheduling block adds a priority class identifier (PCI) and a transmission sequence number (TSN) to the data block and delivers it to a suitable HARQ process. If the transmission of the data block is not successful, the identical data block is retransmitted.
The TFC selection function selects the appropriate transport format of each HS-DSCH when several HS-DSCHs are used for the data transmission. The transmission procedure is described with reference to FIG. 4.
The channel switching block in the MAC-d layer determines the transmission path of the PLC PDU, which is transferred through a Dedicated Traffic Channel (DTCH) or a Dedicated Control Channel (DCCH) from the RLC layer. If the RLC PDU is going to be transmitted to the Dedicated channel (DCH), a header field is added into the PDU and it is transmitted to the physical layer through the DCH. If the HS-DSCH channel is used, the RLC PDU is transmitted to the MAC-c/sh sublayer 162 through a transmission channel multiplexer (T/C MUX). The T/C MUX adds identification information into the header of the PDU in order to identify the logical channel to which each data belongs.
Upon receiving the RLC PDU, the MAC-c/sh sublayer 162 transfers the packet to the MAC-hs sublayer 163. Subsequently, the data transmitted to the MAC-hs sublayer is stored in a buffer of the MAC-hs sublayer 163 and constructed as a data block with a suitable size. The scheduling function determines the size of the data block based on the channel condition. Next, the PCI and TSN are added to the data block, and it is delivered to the HARQ process by the scheduling function.
FIG. 5 illustrates an operation of the scheduling block of the MAC-hs sublayer in Node B. As shown in FIG. 5, the HSDPA scheduler 174 receives information regarding a data priority and an amount of stored data from a storage unit 171, and receives channel status information from the physical layer 173 (S101, S102).
The storage unit 171 is preferably a soft memory in which data can be easily erased and written, and stores the MAC-hs SDUs delivered from an upper layer.
The scheduler 174 controls the operations of the storage unit 171 and the HARQ entity 172 based on the information (steps S103, S104). The scheduler 174 determines the size of the data block, and constitutes the data block, and adds PCI and TSN fields in the data block. Next, the scheduler 174 transmits the corresponding data block to a suitable HARQ process in the HARQ entity, where the data block will be transmitted through the physical layer.
If the data block is successfully transmitted, the corresponding data in the storage unit 171 is deleted based on the feedback information (step S105). At the same time, the HARQ block reports to the scheduler 174 whether or not the data block has been successfully transmitted. Using the transmission result, the scheduler 174 can adjust the transmission of data blocks (step S106).
In general, overall system efficiency and capacity can be determined by an adopted scheduling algorithm. Therefore, the scheduling algorithm should be suitable for characteristics of the provided service.
A scheduling algorithm used in the HSDPA system is currently based on the priority information of each data. The scheduler monitors data in the storage unit of the MAC-hs sublayer, and transmits the data block having the higher priority.
The priority information of each data (MAC-hs SDU) is transferred through a HS-DSCH FP (Frame Protocol) between the Node B and the RNC.
FIG. 6 illustrates a structure of the frame structure of the HS-DSCH FP in the HSDPA system. Referring to FIG. 6, CmCH-PI (Common Channel Priority Indicator) field indicates a priority of the MAC-hs SDUs included in the corresponding data frame, and has values of 0-15. ‘0’ signifies the lowest priority, while “15” signifies the highest priority.
Even though the data priority is the main factor in the HSDPA scheduler, the information is only defined in the specific services. Services supported by the HSDPA include streaming services such as video on demand (VOD) and Audio on demand (AOD), interactive services such as Web browsing and file downloading, and background services such as e-mail and background data downloading. Among these services, the data priority is defined only for interactive services and background services. This has been true so far because the related art streaming services are provided by dedicated resources rather than common resources. However, the HSDPA system provides various services with the common resources.
Additionally, unlike other types of services, the service quality of streaming services is not guaranteed simply by the priority information because data of streaming services are real-time and delay-sensitive. That means there is no need to use the priority information of the data of streaming services.
Typically, real-time data will be lost if the delay of the data exceeds a certain delay limit. In order to reduce the loss of real-time data, the scheduler should take traffic characteristics of streaming services into consideration.
Because the current HSDPA scheduler operates based on the priorities of packets, the HSDPA scheduler may be suitable for non-real-time services (i.e. interactive services and background services). For teal-time services, however, the delay is the most important factor in scheduling. Consequently, there is a problem with the related art scheduler of HSDPA in that is does not take the delay component into account when the supporting services include streaming services.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.